Transition From Soda To Kraft
The delignification reaction during digestion of wood chips in the Kraft pulping process preferentially dissolves the nonfibrous organic lignin "glue" which cements cellulose fibers together in the microstructure of the wood chips. Sodium hydroxide and sodium sulfide are the main pulping constituents (i.e., the active alkali) in white (cooking) liquour used for digestion in the Kraft process.
Historically, sodium hydroxide, alone, was first used by industry for pulping as was then termed the soda process. Unfortunately, the result was excessive losses of the desired cellulose, along with the lignin, caused by chemical degradation of large cellulose molecules due to the "peeling off" reaction. Specifically, small carbohydrate monomers peel off from the large polymeric cellulose molecule, causing rapid formation of water-soluble glucose derivatives and a water-insoluble cellulose molecule of decreased molecular weight. The yield of cellulose pulp was consequently decreased. The Kraft process was then substituted for the soda process despite the resulting disadvantages of Kraft's foul odors, metal corrosion, and explosions caused by the sulfur content of the cooking liquor.
The addition of about 20 percent sodium sulfide to the sodium hydroxide to form white liquor in the Kraft process, results in approximately a five percent increase in pulp yield, and in faster delignification and stronger pulp. Despite its limitations, the Kraft process remains a very versatile, widely-used process, and accounts for approximately 80 percent of the total pulp production in this country. The process continues to be improved in its odor and stands little chance of being replaced in our lifetimes, despite long continued research on alternate pulping schemes. The sodium sulfide apparently serves as both a buffer and pulping catalyst whose functions are still not well-understood.
Most of the sodium hydroxide is consumed during the digestion of wood chips in the Kraft process. The sodium sulfide, by contrast, is depleted to a much lesser degree, which substantiates that it has a role as a "catalyst" in the pulping reactions. About half of the residual active alkali, i.e., that remaining in the spent liquor (now black in color) from digestion, is consequently sodium sulfide, rather than sodium hydroxide, even though the proportions added in white (cooking) liquor were about 4:1 ratio of sodium hydroxide to sodium sulfide.
The concentration of residual active alkali remaining in weak black liquor depends on a number of operating variables in the digestion process. Some of these variables are: (1) sulfidity of the white liquor; (2) wood-to-active alkali ratio with which the digester is charged; (3) type of wood pulped; and (4) the severity of the cook, itself. Downstream from the digester, the active alkali concentration can be changed dramatically by subsequent processing steps on the liquor, such as exposure to hot acidic flue gases from liquor or oil firing in the furnace to eliminate water in the direct contact evaporator. Addition of extraneous by-product streams, such as spent sulfuric acid which is available from the manufacture of chlorine dioxide for bleaching of the pulp, to recover the sodium and sulfur values, can greatly decrease the active alkali concentration present in the black liquor stream as it flows to the chemical recovery furnace. The addition of spent pulping liquors from other pulping processes like sulfite, NSSC, etc., can also modify the active alkali level in the resulting blend. No known mill currently controls the active alkali level in black liquor aside from crude adjustments of its pH. The concentration of active alkali in black liquor consequently varies widely, even though its level is a significant factor in the physical and chemical behavior of the liquor in subsequent processing steps.